There is much interest in converting fermentable renewable sources of carbon to other useful chemicals, such as, but not limited to fuel or fuel additives or specialty or commodity chemicals.
Currently, much industrial fermentation involves the manufacture of ethanol for either chemical or fuel use. However, there is advantage to producing butanol by fermentation. For use in fuel, butanol is superior to ethanol, namely butanol has lower vapor pressure and decreased solubility in water. Hence, it would be desirable to have an economical fermentation process by which butanol could be obtained.
An advantageous butanol fermentation process would encompass a complete, or substantially complete, conversion of sugars to butanol without reaching a butanol titer that causes the rate of butanol production to fall below an undesirable predetermined rate (the “tolerance level”, usually influenced by economic considerations). One way of achieving this goal is to limit sugar loadings to a level whereby batch fermentation does not result in butanol titers that cause the rate of butanol production to fall below the predetermined rate. However, this approach is undesirable because limited sugar loadings result in dilute solutions that are economically undesirable to process. Therefore, there is a need for a process that achieves the aforementioned goal in a way that does not require a limitation on sugar loading.
One means by which a butanol-producing fermentation process might be made more efficient would be to continuously remove butanol from the fermentation medium (broth), so that the tolerance level of the butanol producing microorganism is not reached. This should allow high loadings of sugar to be charged to the fermentation vessel, allowing favorable economics to be achieved. Such a removal process, when associated with fermentation, is generally termed an “In situ Product Removal” process, or an “ISPR” process for short. For an ISPR process to be useful, it needs to be integrated with, compatible with, or easily segregated from, the fermentation process itself. For example, simple distillation of the fermentation broth at atmospheric pressure would not be useful as an ISPR technique because the fermentation microorganisms would most likely be damaged by the temperature in the base of the distillation column.
The present invention represents a means by which In situ Product Removal (ISPR) can be successfully carried out to control butanol concentrations in fermentation broth at or below the tolerance level of the fermentation microorganism. Hence, the current invention is believed to enable butanol fermentation to be carried out using a microorganism, concentrations of sugars and other nutrients which result in favorable economics.
The present invention provides an ISPR process which solves the problem of toxic butanol levels which can destroy a microorganism in a fermentation process by ensuring that butanol concentration in a fermentation broth can be maintained below the tolerance level of the fermentation microorganisms and allowing for subsequent recycle of the fermentation microorganisms back to the fermentation vessel consequently enhancing the efficiency of the process.